As a metal paste for joining or forming metal patterns, there has been developed a composite nanometal paste including composite nanometal particles in which an organic coating layer is formed around a metal core less than or equal to 100 nm. For example, in the embodiment 1 of Japanese Patent Laid-Open No. H10-183207 (patent document 1), it is described the ultra-fine particle in which an organic coating layer of stearic acid group is formed around a silver-metal core. In WO2009/090846 bulletin (patent document 2), the composite nanosilver particles in which an organic coating layer such as alcohol molecule and alcohol derivative is formed around a silver-metal core are disclosed. Furthermore, in the patent document 1 and the patent document 2, it is described that the composite nanometal particles are used for the metal paste. The nanometal particle less than or equal to 100 nm is unstable, because its surface is large, so that the protective layer such as the organic coating layer is needed to exist stably as mentioned above.
In the nanometal particles smaller than or equal to 100 nm, since the melting point is depressed with decreasing of the grain size, the metal paste containing the composite nanometal particles can be sintered at a relatively low temperature, and then it is expected that the composite nanometal paste having the higher joining strength and the preferable electric conductivity is developed.
However, when the metal paste containing the organic coating layer of the metal particle, solvent and so on is sintered by heating, the joining strength and the electric conductivity after sintering are dependent on a ratio of the metal component and organic component in the paste, so that it is demanded that the coating organic component is discharged more efficiently by pyrolysis at the sintering temperature.
In the patent bulletin of JP3630920 (patent document 3), there is described the sintering method of the metal paste that the metal paste in which metal microparticles are dispersed in the organic solvent is preliminarily sintered in an ozone atmosphere under pressure reduction, and the metal paste is really sintered after resolution and removal of the organic solvent. Namely, it is described that the oxygen content adjusted to the predetermined quantity is supplied from outside in order to pyrolyze organic matter contained in the metal paste. Similarly, in the patent bulletin of JP3630552 (patent document 4), it is described the sintering method of the metal paste that the metal paste in which metal microparticles are dispersed in the organic solvent is preliminarily sintered under pressure reduction and the condition that oxygen radical exists, and the metal paste is really sintered in the reducing atmosphere.
In the claim 1 of Japanese Patent Laid-Open No. H11-3617 bulletin (patent document 5), it is described that “a thick-film gold paste which can be sintered under nitrogen atmosphere is characterized by including of a fine powder of gold, a resin for paste adjustment, vehicle and an oxide having a property releasing oxygen”, and in the claim 9 and paragraph 0018 of the document, it is described that the thick-film gold paste is sintered at 500 to 1000° C. under nitrogen atmosphere. Furthermore, in paragraph 0017 of the patent document 5, it is described that “when it is added by small quantity the oxide that is exposed at high temperature under nitrogen atmosphere so that the oxygen is released, it is discovered that a smooth combustion is caused by the released oxygen which assists the oxidation of the resin ingredient”. In paragraph 0014 of the patent document 5, it is described that said thick-film gold paste contains the resin for paste adjustment such as ethyl cellulose, so that “it is verified that a small amount of the resin disappears by exposure at a high temperature not less than 600° C. Namely, in the patent document 5, it is described that when the thick-film gold paste is sintered at 600° C. or higher under the nitrogen atmosphere, the oxide releases the oxygen, which promotes the combustion by assisting oxidation of a resin ingredient for paste adjustment.
In M. Maruyama, et al, “Silver nanosintering: a lead-free alternative to soldering” Appl. Phys. A 93 (2008) 467-470 (non-patent document 1) including the present inventor as one of collaborators, the desorption-resolution process of the organic coating-layer is described in the case using the composite nanometal particle in which an organic coating layer is formed. In the non-patent document 1, it is described that as for sintering in the inert gas, the disappearance of the organic coating layer in the composite nonmetal particle is not the oxidation process in the pyrolysis but the desorption process from a bonding state due to thermal energy. The desorption process of the organic coating layer in the composite nanometal particle under inert gas is kept in slow to a high temperature region in comparison with sintering under the atmosphere, and the temperature threshold at which the organic matter is completely desorbed shifts to the high temperature side more than 70° C.    [Patent Document 1] Japanese Patent Laid-Open No. H10-183207    [Patent Document 2] WO2009/090846    [Patent Document 3] Japanese patent No. 3630920    [Patent Document 4] Japanese patent No. 3690552    [Patent Document 5] Japanese Patent Laid-Open No. H11-3617    [Non-Patent Document 1] M. Maruyama, et al, “Silver nanosintering: a lead-free alternative to soldering” Appl. Phys. A 93 (2008) 467-470